The present invention relates to a radio communication device for use in a radio communication system.
A frequency division multiplex system with small frequency pitches has been widely available due to increasing channel capacity in a mobile communication and in consideration of the signal power in the mobile satellite communication. In such a communication system, a frequency synthesizer with small frequency steps is required.
However, conventional radio communication devices have the following drawbacks:
The first problem will be explained hereinbelow. Since the setting of channel frequencies is carried out by a single frequency synthesizer, if the frequency step is set small, phase noise of the signal is increased. In a synchronous detection demodulation mode required in the system where the channel has a narrow band and the power is limited, an influence of phase noise is more liable and thus more serious.
Now, the second problem will be explained hereinbelow. For reduction in size of a communication terminal in the radio communication system, a TDMA/FDMA system in combination of a time division multiplex (TDMA) system and the frequency division multiplex (FDMA) system has been widely available since a duplexer can be constituted by a simple radio-frequency switch. In the TDMA/FDMA system, a frequency synthesizer capable of high-speed frequency switching is required for preventing interference between users. However, reduction in frequency pitch and reduction in frequency switching time belong to conflicting requirements and are difficult to achieve from a technical point of view.
Finally, the third problem will be explained hereinbelow. For solving the foregoing problems, a method has been taken wherein the frequency synthesizer has a double-loop structure. Such a double-loop type frequency synthesizer is described in "Frequency Synthesizer Design Handbook, page 289" by James A. Crawford, Artech House (hereinafter referred to as "Prior Art 1").
The method of using the double-loop structure gives a tentative solution from a technical point of view, which, however, requires a complicated device structure due to the double-loop structure.